1. Field of the Invention
The present invention relates to a touch panel, and more particularly, to an optical touch panel.
2. Description of the Related Art
Touch panels are now used which can give an instruction to an apparatus when an interface image projected onto a display is touched. While there are a variety of specific operating principles for touch panels, touch panels are requested which have sufficient mechanical strength and high transmittance. In response to such requests, in order to provide mechanical strength, various optical touch panels have been proposed which use an optical detection mechanism. Further, in optical touch panels, in order to reduce the effect of contamination of the detection mechanism, structures have been proposed for preventing light emitting surfaces for use in detection and detection sensor light receiving surfaces from being exposed on the surface.
FIG. 12 is a diagram illustrating an optical touch panel with a light emitting surface and a light receiving surface unexposed on the surface, which is disclosed in U.S. Pat. No. 4,868,551. According to U.S. Pat. No. 4,868,551, a detection surface 701 composed of a light guide plate is disposed at a front of a CRT that is a scanning display device, and light for detection is scanning and directed to the light guide plate. Then, the reflected light is detected by a light receiving element 702, and the time when the light returns is used to calculate where on the scanning lines the light is located at that time so as to detect the location of contact of an object.
FIG. 13 is a diagram illustrating an optical touch panel with a light emitting surface and a light receiving surface unexposed on the surface, which is disclosed in Japanese Patent Laid-Open No. 2000-259347. A light guide plate is provided on a field sequential display apparatus such as a liquid crystal display panel, and a light source and a line sensor for detection are disposed on side faces of the light guide plate. Then, light 801 is supplied from the light source on the side face of the light guide, and a detected object 802 composed of a material such that the light is absorbed by the object to be detected or guided light toward the object is used as an indicating device for a touch panel. Optical signals, which are changed by absorbing light irradiation from the side face when the indicating device touches the surface of the light guide plate, are detected by the line sensor 803 for detection.
However, in the touch panel in U.S. Pat. No. 4,868,551, in principle, all of the light propagated in the panel is focused on the photodetector for detection Thus, when such a touch panel is used in an environment where there is external light, an amount of light to which has been added the external light over the entire surface reaches the single point that is the receiving element.
More specifically, background components of signals proportional to the product of the width and length of the face constituting the touch panel appear, the ratio of the signal to be detected to the background component (hereinafter, this index is referred to as contrast) is significantly decreased, and it becomes difficult to detect the position indicated.
Further, there is fundamentally an essential problem that the display disposed under the panel always has to operate in a scanning manner, and displays that operate in a non-scanning manner are thus inoperative because the operating principle is not able to be applied.
Moreover, for the touch panel in Japanese Patent Laid-Open No. 2000-259347, indicating devices which are not dedicated indicating devices, such as, for example, a finger, are not be able to be used. Since a finger is not an ideal light absorber, most of the light is diffused or reflected by the epidermis or dermis of the finger, and therefore most of the light is returned to the light guide panel of the touch panel. Therefore, in principle, the contrast will be lowered. Further, the presence of external light further reduces the essentially already low contrast, thus making it more difficult to detect the position indicated.
However, for touch panels for mobile devices such as digital cameras and cellular phones, which are assumed to be operated with a variety of indicating devices such as a finger or a gloved finger in a variety of environments such as outdoors, it is strongly desired to reduce the influence of external light on the touch panels. Further, it is strongly desired for the touch panels to operate precisely even when indicating devices such as a finger or a gloved finger are used.